There's a lot of variation in doc comments and internal code about
whether chmod/0o777-style permissions are called `mode` or `perm`. (For
example, mkdir and writeFile choose differently.)
Had proposed earlier to go consistently with `perm`, but on balance devs
prefer to go with `mode`.
Rewrite "testPerm" helper function used for testing of internal
runtime code. It's been renamed to "unitTest" and provides API that
is extensible in the future by accepting optional "UnitTestOptions"
argument. "test" helper was also removed and replaced by
overloaded version of "unitTest" that takes only function argument.
"UnitTestOptions" currently supports "perms" and "skip"
options, where former works exactly as first argument to "testPerm"
did, while the latter allows to conditionally skip tests.
